A complicated relationship : Glycosylation, Ca(II), and primary sequence affect the interactions and kinetics between two model mollusk shell intracrystalline nacre proteins

The formation of the mollusk shell requires the participation of proteins, many of which may be interactive with one an-other. We examined a model protein pair system from the mollusk Haliotis rufescens wherein we probed the interactions between recombinant forms of two major nacre layer proteins, AP7, and the glycoprotein, AP24. Here, the focus was on the impact that AP24 glycosylation and primary sequence had on AP24-AP7 binding. We find that both the glycosylated and non-glycosylated variants of AP24 bound to AP7 but with different quantities, kinetics, and internal rearrangements. Moreover, the binding of AP7 with non-glycosylated and glycosylated AP24 was found to be Ca(II) dependent and independent, respectively. Yet both variants of AP24 combine with AP7 to form hybrid hydrogel particles that are similar in their physical properties. Thus, AP7 and AP24 protein sequences are interactive and form hydrogels, but the interactions are tuned by glycosylation and Ca(II). These features may have an impact on nacre matrix formation.